In butt welding metallic sheets such as described in German patent document 3,713,527 it is necessary to avoid the formation of any gaps at the butted edges being welded, whether they are of two different workpieces or opposite edges of a single workpiece as in a tube-forming operation Accordingly the edges are formed as straight and square as possible, typically by shearing, and then are pressed together and held before welding heat is applied. If a gap is left that is as small as one-tenth the workpiece thickness the result can be a mechanically weak joint that in certain applications will create a leak. When the gap is larger a laser beam used to apply the heat might pass between the workpieces, preventing them from being fused together at all.
Accordingly it is known from German patent document 3,901,319 to overlap the workpiece edges slightly by a distance equal to the maximum possible variation of straightness of these edges. The overlapped edges are then flattened together by a roller or the like before welding, producing in regions of some overlap a workpiece thickness equal to 1.3 to 1.5 times the material thickness. Trying to reduce the thickness further damages the workpieces. This procedure therefore produces a thick unsightly weld where, due to varying material thickness, actual fusion will normally vary along the weld, being less in the thick regions and more in the regions where there is little or no overlap.